The invention relates to a predictive encoder for still pictures, comprising a quantizer whose characteristic curve is variable, an output buffer from which the encoded picture data are read and transmitted at a constant bit rate, and a subtractor and a picture memory for obtaining prediction values.
A hybrid encoder for video signals is known from DE 36 13 343 A1 and corresponding U.S. Pat. No. 4,825,285 in which the data of a video picture are subdivided into equally large data blocks by means of a preprocessing unit. Each data block comprises the data of a square picture section of 64 pixels. For example, a data block may comprise the chrominance values of the first 8 pixels of the first 8 lines of a video picture. The preprocessing unit also assigns an identification to each data block, which indicates the position of the block within a video picture. Data blocks of successive video pictures having the same identification are designated as corresponding data blocks. A subtractor compares each data block with the corresponding data block of the previous video picture. The corresponding data block of the previous video picture is present at the output of a first picture memory. The difference block produced by the subtractor is subjected in a first operation unit to a matrix transform (the data blocks are treated as square number matrixes). This transform is eliminated in a feedback path by a second operation unit and the regained difference block is applied to a first input of an adder. The second input of the adder is connected to the output of the first picture memory, while the output of the adder is connected to the data input of the first picture memory.
The main object of the known encoder is to encode the video data from a video data source with a minimum possible loss of information into a signal with a minimum possible bit rate. In this process two encoding principles are used, hence the name hybrid encoder: the interframe principle in which the correlation (by block differentiation) between time-sequential video pictures (used in this respect for frames and fields) is utilized and the intraframe principle in which the correlation of the video data (by the matrix transform) within a video picture is utilized.
The known arrangement is specially designed for the transmission of so-called moving pictures, thus of sequences of video pictures comprising data which are different from picture to picture because the imaged objects move relative to the video camera. To reduce the transmission bit rate, for example to 2 Mbit/s, the abovementioned preprocessing unit also comprises a device for sub-sampling the video pictures supplied by a video camera. These video pictures comprise 556 lines and 720 columns, i.e. 400320 pixels. After the sub-sampling operation the number of pixels amounts to only a quarter of the original number of pixels. This automatically results in a loss of quality of the video pictures supplied by a video camera.